A description of fly ash, its source and previous disposition is given in column 1 of U.S. Pat. No. 3,328,180, hereby incorporated by reference.
Various uses for fly ash are well-known in the construction (and particularly concrete) industry. Uses include production of aggregates (by various processes) for use in concrete products, raw feed to cement production processes, and direct replacement of a portion of the cement used in concrete products. This invention addresses the partial direct replacement of cement.
Various fly ash properties are known to limit its usefulness to replace cement. Chief among these are high carbon content and large particle size. Other undesirable properties include excess magnetic iron compounds, excess cenospheres, and low pozzolanity. Known processes have been applied to improve some of these characteristics with various economic results. For example, particle size may be reduced by pulverization, sieving or classification. Likewise, excess magnetic iron may be removed by magnetic separation and excess cenospheres by flotation. Although low pozzolanity may be improved by these methods, it is sometimes associated with peculiarities in the fly ash chemistry and not subject to improvement by known processes.
This invention deals with reduction of carbon in the fly ash. This has been accomplished in the past by several methods. However, the coal-fired furnace modifications, classification by particle size, and electrostatic separation methods as well as the wet flotation method in U.S. Pat. No. 4,121,945 accomplish carbon reduction by processes clearly different from those of this invention. Only this invention and the processes taught in U.S. Pat. No. 3,979,168, a report from Vliegasunie B. V. (Dutch Fly Ash Corporation, undated report, page 6, paragraph 3.3, published in the mid 1980's) and U.S. Pat. No. 4,705,409 reduce ash carbon content by oxidizing the carbon.
Burning of carbon is known. However, when the carbon is highly diluted by inert compounds and is not accompanied by volatile compounds as in fly ash the combustion becomes very difficult. The critical factors in obtaining such combustion are residence time, reaction temperature, and oxygen availability. Minimum ignition temperature and oxygen requirements are relatively well understood. A proper combination of operating conditions for dry fluid bed oxidation and particularly minimum residence time were previously unknown for carbon in fly ash.
In early work, two types of transport reactors were tested with residence times on the order of 1 to 15 seconds. By transport reactor, is meant a reactor in which all of the reactants travel together at more or less the same speed. In this early work, the large volume of air necessary to provide sufficient oxygen for carbon burnout was used to transport the reacting fly ash from inlet to discharge points. No substantial carbon burnout was detected.
Fluid bed reactors are well-known for, among other things, their ability to provide extended residence time for certain reactions Fluid beds are loosely divided into bubbling bed and circulating bed types. In the bubbling bed, the solid material stays substantially in place in the bed while the gaseous (or sometimes liquid) material travels upward through it In the circulating bed type, velocity of the fluid is greatly increased so that the bed becomes very dilute and most or all of the solid material is repeatedly ejected from the bed, separated from the fluid and reinjected into the bed.
Circulating fluid beds are known to be undesirable for very fine solids (such as fly ash) due to the economic difficulty of repeatedly separating fine solids from the fluid stream.
This invention has overcome the problems of prior art processing methods by discovering how to economically use a dry bubbling fluid bed reactor. By "dry" is meant the fluid bed is substantially free of any liquid.
It had been observed in the fluid catalyst bed art that dense beds of powder could be maintained at much higher gas velocities than the calculated settling velocity of individual particles. F. A. Zenz notes this in the Handbook of Powder Science and Technology (edited by M. E. Fayed and L. Otter) Chpt. 10, p. 464 (1984 Van Nostrand Reinhold Co.). It was not known, however, whether a dry bubbling bed reactor, using fly ash particles instead of catalyst particles, could be used economically to reduce fly ash carbon. Because fly ash has a much lower value than catalyst fines, acceptable economics would be much different. In addition, the expected allowable velocity would be on the order of 0.01 ft/sec, the single particle terminal settling velocity expected for the fine fly ash particles to prevent excess transport of material from the bubbling bed. A reactor using this velocity would have a huge plan area to pass the required air quantity and would be clearly uneconomic. It has now been discovered that bed velocity can be increased approximately two orders of magnitude without departing significantly from the bubbling bed regime, even for non-catalytic bubbling fluid beds using the fine fly ash particles as the bed of particles. Use of the high bed velocity allows reduction of bed plan area by a factor of approximately 100 with obvious economic benefits.